Match scan capture groups behaviour to Ruby

[ocvit]

# ruby
"ruby:123, rust:456".scan(/\w+:\d+/)
# => ["ruby:123", "rust:456"]

# before
RustRegexp.new('\w+:\d+').scan("ruby:123, rust:456")
# => [["ruby:123"], ["rust:456"]]

# after
RustRegexp.new('\w+:\d+').scan("ruby:123, rust:456")
# => ["ruby:123", "rust:456"]